1. Field of the Invention
The present invention relates to an X-ray apparatus for panoramic tomography comprising an X-ray source and X-ray detection means and provided with a moving mechanism for controlling movements for panoramic imaging, including translation and rotation of the X-ray source and of the detection means with respect to an object to be examined.
2. Description of the Prior Art
In known panoramic X-ray apparatus for dental photography the X-ray beam is turned around the patient's head in such a way that the dental arch becomes photographed as a flat picture on a moving film.
In order to get a sharp picture of the object and to make structures in front of the object and behind it invisible by "fogging" out of focus, the transverse velocity of the film with respect to the ray beam is kept equal to the sweep velocity of the ray beam in the object taking in consideration the magnification ratio. The magnification is determined by the ratio of the distance between the focus and the film to the distance between the focus and the object.
The thickness of the layer being photographed sharply is directly proportional to the distance of the the instantaneous center of rotation from the object plane, and inversely proportional to the magnification and to the width of the ray beam. From a point of view of how the object will be represented it is important only how the X-ray, focuses and how the object and the film plane are mutually registered.
In order to realise an orthogonal radiogram of the teeth and the jaw, typical to panoramic tomography X-ray equipment, a thin X-ray beam is generated by penetrating the relevant parts of a patient. The anode current and the anode voltage of an X-ray source are controlled according to the properties of the patient while the X-ray beam is rotated around the patient along a path such that the X-ray beam meets the jaw and the teeth as perpendicularly as possible. To this end, the centre of rotation of the X-ray beam is usually translated during X-ray emission, either into a limited number of of fixed positions or continuously along an adapted path. This translation is usually achieved by means of a cam or equivalent mechanism. This implies that three different kinematic axes are involved in the movement of the X-ray beam to wit: the angle of rotation and the two cartesians X and Y of the centre of rotation. A film and a film cartridge, located on the opposite side of the patient with respect to an X-ray tube, are moved with such a speed that a desired layer of the patient's chinbone and dental system are represented on the film sharply.
It is also known to use a separate motor for the film cartridge motion; usually this motor is of constant-speed type, and the gear ratio of the film cartridge is mechanically changed by means of forced control provided by the rotary movement.
A drawback of known control systems based on mechanically interconnected motion speeds is that the ratio of the motion speeds is mechanically fixed and cannot be changed in the different stages of the exposure; therefore only one type of jaw profile can be photographed sharply with these devices. As known, human jaws vary, and therefore it has been necessary to make compromises in the known devices, for example such that a so called average jaw profile in the photographed sharp layer in order to have different jaws be photographed at least satisfactorily.
For eliminating these drawbacks a separate film cartridge transport motor is provided in a further known embodiment, the speed of which can varied as before with control provided by the rotary motion but also separately so that the layer being photographed sharply can when desired be changed within certain limits thus aiming at getting as good panoramic photograph as possible of each jaw shape.
In the last-mentioned known equipment, in which the rotary motion and the film cartridge movement have their own motors, the rotary motion motor is a constant-speed motor; in consequence it has been possible to change the layer being photographed sharply only by changing the speed of the film cartridge. This procedure has the inevitable consequence that the exposing time will change directly proportionally to the film cartridge transport speed, and the picture will not be evenly exposed; instead, it will have lighter and darker stripes depending on to what direction the sharply-represented layer has been corrected with respect to the basic shape determined by the mechanical system. Similarly, in these known systems with two separate motors it has not been possible to make the necessary correction in order to compensate the absorption of radiation caused by the cervical spine, because always at least one of the motors is of the constant-speed type. Therefore a compensation is introduced by changing the intensity of the X-ray beam, but as a milli-ampere-regulation with which the intensity is controlled is too slow due to properties of the glow filament of the X-ray tube, one must change the intensity by changing the anode voltage of the X-ray tube. Depending on the properties of the patient and intensifying screens used, the maximum appropriate contrast is achieved with a certain anode voltage; deviating from this voltage results in worse contrast. Therefore in an apparatus as disclosed in EP No. 215757 a control system is introduced on the basis of control data and program stored into it, arranged to control both the speed of a rotary motion motor of an arm and the speed of a film transport motor and the distribution of these speeds during an exposure sequence independently of each other in order to obtain sharp images with suitable exposing time photograph dental systems of varying sizes and shapes with a panoramic device. The use of a mechanically fixed movement by the motor drive of the arm still restricts an exact panoramic imaging.